Manipulation of plant senescence

ABSTRACT

The present invention relates to methods of manipulating senescence in plants. The invention also relates to vectors useful in such methods, transformed plants with modified senescence characteristics and plant cells, seeds and other parts of such plants.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from International Patent Application PCT/AU 01/01092, entitled “Manipulation of Plant Senescence Using An MYB Gene Promoter and Cytokinin Biosynthesis Genes,” filed Aug. 30, 2001, which claims priority from Australian Patent Application PQ 9946, entitled “Manipulation of Plant Senescence Using An MYB Gene Promoter and Cytokinin Biosynthesis Genes,” filed Sep. 6, 2000; the contents of which are incorporated by reference herein in its entirety.

The present invention relates to methods of manipulating senescence in plants. The invention also relates to vectors useful in such methods, transformed plants with modified senescence characteristics and plant cells, seeds and other parts of such plants.

Leaf senescence involves metabolic and structural changes in cells prior to cell death. It also involves the recycling of nutrients to actively growing regions.

The regulation of plant and plant organ senescence by cytokinins has important agricultural consequences. Elevated cytokinin levels in leaves tend to retard senescence. A number of promoters have been used to regulate the expression of the ipt gene, whose product (isopentenyltransferase) catalyses a key step in cytokinin synthesis. However, in general, transgenic plants over-expressing the ipt gene have been reported to have retarded root and shoot growth, no root formation, reduced apical dominance, and reduced leaf area.

It is an object of the present invention to overcome, or at least alleviate, one or more of the difficulties or deficiencies associated with the prior art.

In one aspect, the present invention provides a method of manipulating senescence in a plant, said method including introducing into said plant a genetic construct including a myb gene promoter, or a functionally active fragment or variant thereof, operatively linked to a gene encoding an enzyme involved in biosynthesis of a cytokinin, or a functionally active fragment or variant thereof.

The manipulation of senescence relates to the plant and/or specific plant organs. Senescence of different plant organs, such as leaves, roots, shoots, stems, tubers, flowers, stolons, and fruits may be manipulated. The manipulation of plant and plant organ senescence may have important agricultural consequences, such as increased shelf life of e.g. fruits, flowers, leaves and tubers in horticultural produce and cut flowers, reduced perishability of horticultural crops, increased carbon fixation in senescence-retarded leaves leading to enhanced yields, enhanced biomass production in forage plants, enhanced seed production, etc.

“Manipulating senescence” generally relates to delaying senescence in the transformed plant relative to an untransformed control plant. However, for some applications it may be desirable to promote or otherwise modify senescence in the plant. Senescence may be promoted or otherwise modified for example, by utilizing an antisense gene.

An effective amount of said genetic construct may be introduced into said plant, by any suitable technique, for example by transduction, transfection or transformation. By “an effective amount” is meant an amount sufficient to result in an identifiable phenotypic trait in said plant, or a plant, plant seed or other plant part derived therefrom. Such amounts can be readily determined by an appropriately skilled person, taking into account the type of plant, the route of administration and other relevant factors. Such a person will readily be able to determine a suitable amount and method of administration. See, for example, Maniatis et al, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, the entire disclosure of which is incorporated herein by reference.

Preferably the myb gene promoter is a myb32 promoter. Preferably the myb gene promoter is from Arabidopsis, more preferably Arabidopsis thaliana. Most preferably the myb gene promoter includes a nucleotide sequence selected from the group consisting of the sequence shown in FIG. 1 hereto (Sequence ID No: 1) and functionally active fragments and variants thereof.

A suitable promoter is described in Li et al., Cloning of three MYB-like genes from Arabidopsis (PGR 99–138) Plant Physiology 121:313 (1999), the entire disclosure of which is incorporated herein by reference.

By “functionally active” is meant that the fragment or variant (such as an analogue, derivative or mutant) is capable of manipulating senescence in a plant by the method of the present invention. Such variants include naturally occurring allelic variants and non-naturally occurring variants. Additions, deletions, substitutions and derivatizations of one or more of the nucleotides are contemplated so long as the modifications do not result in loss of functional activity of the fragment or variant. Preferably the functionally active fragment or variant has at least approximately 80% identity to the relevant part of the above mentioned sequence, more preferably at least approximately 90% identity, most preferably at least approximately 95% identity. Preferably the fragment has a size of at least 10 nucleotides, more preferably at least 15 nucleotides, most preferably at least 20 nucleotides.

The gene encoding an enzyme involved in biosynthesis of a cytokinin may be of any suitable type. Preferably the gene is an isopentenyl transferase (ipt) gene. Preferably the gene is from Agrobacterium, more preferably Agrobacterium tumefaciens. Most preferably the gene includes a nucleotide sequence selected from the group consisting of the sequence shown in FIG. 2 hereto (Sequence ID No: 2) and functionally active fragments and variants thereof.

By “functionally active” is meant that the fragment or variant (such as an analogue, derivative or mutant) is capable of manipulating senescence in a plant by the method of the present invention. Such variants include naturally occurring allelic variants and non-naturally occurring variants. Additions, deletions, substitutions and derivatizations of one or more of the nucleotides are contemplated so long as the modifications do not result in loss of functional activity of the fragment or variant. Preferably the functionally active fragment or variant has at least approximately 80% identity to the relevant part of the above mentioned sequence, more preferably at least approximately 90% identity, most preferably at least approximately 95% identity. Such functionally active variants and fragments include, for example, those having nucleic acid changes which result in conservative amino acid substitutions of one or more residues in the corresponding amino acid sequence. Preferably the fragment has a size of at least 10 nucleotides, more preferably at least 15 nucleotides, most preferably at least 20 nucleotides.

The genetic construct may be introduced into the plant by any suitable technique. Techniques for incorporating the genetic constructs of the present invention into plant cells (for example by transduction, transfection or transformation) are well known to those skilled in the art. Such techniques include Agrobacterium mediated introduction, electroporation to tissues, cells and protoplasts, protoplast fusion, injection into reproductive organs, injection into immature embryos and high velocity projectile introduction to cells, tissues, calli, immature and mature embryos, and combinations thereof. The choice of technique will depend largely on the type of plant to be transformed, and may be readily determined by an appropriately skilled person.

Cells incorporating the genetic construct of the present invention may be selected, as described below, and then cultured in an appropriate medium to regenerate transformed plants, using techniques well known in the art. The culture conditions, such as temperature, pH and the like, will be apparent to the person skilled in the art. The resulting plants may be reproduced, either sexually or asexually, using methods well known in the art, to produce successive generations of transformed plants.

The method of the present invention may be applied to a variety of plants, including monocotyledons [such as grasses (forage and turfgrasses), corn, oat, wheat and barley)], dicotyledons [such as Arabidopsis, tobacco, clovers (e.g. white clover, red clover, subterranean clover), alfalfa, canola, vegetable brassicas, lettuce, spinach] and gymnosperms.

In a second aspect of the present invention there is provided a vector capable of manipulating senescence in a plant, said vector including a myb gene promoter, or a functionally active fragment or variant thereof, operatively linked to a gene encoding an enzyme involved in the biosynthesis of a cytokinin, or a functionally active fragment or variant thereof.

In a preferred embodiment of this aspect of the invention, the vector may further include a terminator; said promoter, gene and terminator being operatively linked.

By “operatively linked” is meant that said promoter is capable of causing expression of said gene in a plant cell and said terminator is capable of terminating expression of said gene in a plant cell. Preferably, said promoter is upstream of said gene and said terminator is downstream of said gene.

The vector may be of any suitable type and may be viral or non-viral. The vector may be an expression vector. Such vectors include chromosomal, non-chromosomal and synthetic nucleic acid sequences, eg. derivatives of plant viruses; bacterial plasmids; derivatives of the Ti plasmid from Agrobacterium tumefaciens; derivatives of the Ri plasmid from Agrobacterium rhizogenes; phage DNA; yeast artificial chromosomes; bacterial artificial chromosomes; binary bacterial artificial chromosomes; vectors derived from combinations of plasmids and phage DNA. However, any other vector may be used as long as it is replicable or integrative or viable in the plant cell.

The promoter, gene and terminator may be of any suitable type and may be endogenous to the target plant cell or may be exogenous, provided that they are functional in the target plant cell.

A variety of terminators which may be employed in the vectors of the present invention are also well known to those skilled in the art. The terminator may be from the same gene as the promoter sequence or a different gene. Particularly suitable terminators are polyadenylation signals, such as the CaMV 35S polyA and other terminators from the nopaline synthase (nos) and the octopine synthase (ocs) genes.

The vector, in addition to the promoter, the gene and the terminator, may include further elements necessary for expression of the gene, in different combinations, for example vector backbone, origin of replication (ori), multiple cloning sites, spacer sequences, enhancers, introns (such as the maize Ubiquitin Ubi intron), antibiotic resistance genes and other selectable marker genes [such as the neomycin phosphotransferase (nptll) gene, the hygromycin phosphotransferase (hph) gene, the phosphinothricin acetyltransferase (bar or pat) gene], and reporter genes (such as beta-glucuronidase (GUS) gene (gusA)]. The vector may also contain a ribosome binding site for translation initiation. The vector may also include appropriate sequences for amplifying expression.

As an alternative to use of a selectable marker gene to provide a phenotypic trait for selection of transformed host cells, the presence of the vector in transformed cells may be determined by other techniques well known in the art, such as PCR (polymerase chain reaction), Southern blot hybridisation analysis, histochemical assays (e.g. GUS assays), thin layer chromatography (TLC), northern and western blot hybridisation analyses.

Those skilled in the art will appreciate that the various components of the vector are operatively linked, so as to result in expression of said gene. Techniques for operatively linking the components of the vector of the present invention are well known to those skilled in the art. Such techniques include the use of linkers, such as synthetic linkers, for example including one or more restriction enzyme sites.

In a further aspect of the present invention there is provided a transgenic plant cell, plant, plant seed or other plant part, with modified senescence characteristics. Preferably the transgenic plant cell, plant, plant seed or other plant part is produced by a method according to the present invention.

The present invention also provides a plant, plant seed or other plant part derived from a plant cell of the present invention.

The present invention also provides a plant, plant seed or other plant part derived from a plant of the present invention.

The present invention will now be more fully described with reference to the accompanying examples and drawings. It should be understood, however, that the description following is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above.

In the figures:

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 shows the nucleotide sequence of the promoter from myb32 gene (atmyb32) from Arabidopsis thaliana (Sequence ID No: 1).

FIG. 2 shows the nucleotide sequence of the isopentenyl transferase (ipt) gene from Agrobacterium tumefaciens (Sequence ID No: 2).

FIG. 3 shows PCR and Southern DNA analysis of atmyb32::ipt transgenic white clover (Trifolium repens) plants. a) The T-DNA region of patmyb32:ipt showing restriction enzyme sites and location of the probes used for Southern hybridization analysis. b) Ethidium bromide stained 1% agarose gel of the PCR amplified 599 bp nptll and 583 bp ipt products. c) Southern blot hybridization with HindIII digested total genomic DNA isolated from PCR positive white clover plants hybridized with the ipt probe. d) Southern blot hybridization with HindIII digested total genomic DNA isolated from PCR positive white clover plants hybridized with the nptll probe. Lanes 1–2: two independent kanamycin resistant cv. Haifa regenerants, code: Hmi01, Hmi08 respectively; Lanes 3–12: twelve independent kanamycin resistant cv. Irrigation regenerants, codes: Imi06, Imi07, Imi08, Imi09, Imi10, Imi11, Imi12, Imi14, Imi16, Imi18 respectively; Lane C: non-transformed white clover; Lane P: positive control plasmid patmyb32ipt.

FIG. 4 shows RT-PCR analysis of ipt mRNA expression in atmyb32::ipt transgenic white clover (T. repens) plants. Lane 1–11 are samples from 11 independent transgenic lines with corresponding plant codes as in FIG. 4.8; Lane C, Control non-transformed plant; Lane P, plasmid as positive control. Total RNA was isolated from leaf tissues. Total RNA (13 μg) was used for each reverse transcription reaction and ⅕ of RT product was amplified by PCR. DNA products on the gel on the right were amplified by 2×30 cycles intensive PCR. No reverse transcriptase was added to the corresponding RT-PCR reaction loaded into alternate lanes.

FIG. 5 shows a senescence bioassay of excised leaves from atmyb32::ipt transgenic white clover (T. repens) plants. At least 30 leaves were collected from each line from similar positions on stolons of plant lines. A. The number of yellowing leaves as a fraction of the total number of excised leaves. B. Typical appearance of leaves kept on water under light for two weeks. Key to plant lines: HC, IC and Hmg, Img, non-transformed and atmyb32::gusA transgenic plants (cv. Haifa and Irrigation) respectively; 01 and 08, atmyb32::ipt transgenic Haifa lines Hmi01 and Hmi08 respectively; 11, 12, 16 and 18 atmyb32::ipt transgenic Irrigation lines Imi11, Imi12, Imi16 and Imi18 respectively.

FIG. 6 shows A) General plant morphology, B) Normal shoot development, and C) Normal root development in atmyb32:::ipt transgenic white clover (T. repens) (right) plants compared to control plants (left).

EXAMPLES Example 1 Production of atmyb32::ipt Transgenic Plants

Transgenic white clover plants (Trofolium repens cv. Haifa and Irrigation) were produced by Agrobacterium-mediated transformation using a binary vector carrying the chimeric atmyb32::ipt gene (FIG. 3 a). The transgenic plants were screened by PCR using ipt and nptll primers (FIG. 3 b). HindIII digested genomic DNA samples subjected to Southern DNA hybridization analysis showed that the DNA fragments greater than 4.4 kb were detected in all lanes by both ipt and nptll probes, demonstrating the presence and integration of full-length T-DNA into the white clover genome (FIG. 3). Transgenic lines Hmi01, Imi06, Imi11, and Imi18 (Lane 1, 3, 5, 8 and 12 respectively) appeared to have a single copy of full-length T-DNA integrated in the genome. Other transgenic lines had multiple copies of the atmyb32::ipt transgene.

Example 2 Expression of atmyb32::ipt Gene in Transgenic Plants

The expression of the atmyb32::ipt transgene in transgenic white clover (T. repens) plants was assessed by RT-PCR. The ipt mRNA was detected in leaf tissues of all atmyb32::ipt transgenic white clover plants examined, with varying levels of PCR products detected (FIG. 4).

Example 3 Delayed Detached Leaf Senescence in atmyb32::ipt Transgenic Plants

Experiments were performed to assess detached leaf senescence of atmyb32::ipt transgenic plants. Rapid yellowing was observed in detached leaves from non-transformed and atmyb32::gusA transgenic white clover plants of both cultivars within one week. Transgenic lines Hmi01, Hmi08, Imi16 and Imi18 showed delayed senescence while Imi11 and Imi12 showed no sign of yellowing by the end of 7 days. After two weeks, the leaves of all atmyb32::ipt transgenic plants were much greener than those of non-transformed and atmyb32::gusA control transgenic plants (FIG. 5). The degree of senescence in excised leaves was in the order HC, Hmg>Hmi01>Hmi08 for cv. Haifa, and IC and Img>Imi16>Imi18>Imi11 and Imi12 for cv. Irrigation. HC is Haifa untransformed control, Hmg is Haifa atmyb32::gusA control, IC is Irrigation untransformed control, Img is Irrigation atmyb32::gusA control. Hmi01, Hmi08, Imi16, Imi18, Imi11 and Imi12 are independent atmyb32::ipt transgenic white clover plants from the cultivar Haifa (H) and Irrigation (I), respectively.

Example 4 Normal Plant Morphology and Root Development in atmyb32::ipt Transgenic Plants

Normal plant morphology as well as normal shoot and normal root development was observed in atmyb32:ipt transgenic white clover plants (FIG. 6), thus indicating that the regulated expression of the ipt gene under control of the atmyb32 promoter did not negatively affect neither rooting nor apical dominance of the transgenic white clover plants (Table 1).

TABLE 1 Transformant Cultivar Construct ipt copy No Phenotype Hmi01 Haifa atmyb32::ipt 1 Normal Hmi08 Haifa atmyb32::ipt >3 Normal Imi06 Irrigation atmyb32::ipt 1 Normal Imi07 Irrigation atmyb32::ipt 3 Normal Imi09 Irrigation atmyb32::ipt >3 Normal Imi10 Irrigation atmyb32::ipt >3 Normal Imi11 Irrigation atmyb32::ipt 1 Normal Imi12 Irrigation atmyb32::ipt 2 Normal Imi16 Irrigation atmyb32::ipt 2 Normal Imi18 Irrigation atmyb32::ipt 1 Normal Normal plant morphology and normal rooting was observed in ten independent atmyb32::ipt transgenic white clover lines analyzed. Estimated ipt gene copy numbers in the ten independent atmyb32::ipt transgenic white clover lines are shown.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Finally, it is to be understood that various alterations, modifications and/or additions may be made without departing from the spirit of the present invention as outlined herein.

It will also be understood that the term “comprises” (or its grammatical variants) as used in this specification is equivalent to the term “includes” and should not be taken as excluding the presence of other elements or features.

Documents cited in this specification are for reference purposes only and their inclusion is not an acknowledgement that they form part of the common general knowledge in the relevant art. 

1. A method of manipulating senescence in a plant, plant cell, plant seed or other plant part, said method comprising introducing into said plant, plant cell, plant seed or other plant part a genetic construct comprising the myb gene promoter of SEQ ID NO:1, operably linked to the isopentenyl transferase (ipt) gene of SEQ ID NO:2.
 2. The method according to claim 1, wherein said genetic construct is introduced into said plant, plant cell, plant seed or other plant part by transduction, transfection or transformation of plant cells.
 3. The method according to claim 2, wherein said plant cell comprising the genetic construct is selected and then cultured to regenerate a transformed plant.
 4. A vector comprising the myb gene promoter of SEQ ID NO:1 operably linked to the isopentyl transferase (ipt) gene of SEQ ID NO:2.
 5. A transgenic plant cell, plant, plant seed or other plant part with modified senescence characteristics, said plant cell, plant, plant seed or other plant part comprising the vector according to claim
 4. 6. A transgenic plant cell, plant, plant seed or other plant part with modified senescence characteristics, said plant cell, plant, plant seed or other plant part produced by the method according to claim
 1. 